The Planets

Some estimates suggest this water world was far from fleeting, a blue planet just like our own that could have been sustained for around 2 billion years and perhaps only disappeared some 700 million years ago. It’s a tantalising thought that such a similar world to our own existed for so long with liquid water on its surface. We know life took hold quickly on our own blue planet, within half a billion years of the Earth being formed, so there seems good reason to suspect that if Venus really was as wet as the models predict, it too could have sprung into life. Exactly what went on in the long-lost rivers and oceans of Venus is yet to be discovered; hidden behind the clouds, we have not yet been back to search for any signs that life ever took hold here. Our exploratory attentions have turned to Mars as a planet that not only has a fertile past but is also a possible target for human colonisation in the future. We know for certain that no life (at least no life we understand) could exist on Venus today, and perhaps even the evidence of any biology on that long-lost water world has long ago vanished under the oppressive heat, rampant volcanism and extreme pressures of the planet today. So where did all that water go? Understanding this requires an exploration of the differences between Earth and Venus, as well as the similarities.

© LIBRARY OF CONGRESS / SCIENCE PHOTO LIBRARY

Venus has fascinated scientists for centuries; this diagram was drawn by Nicholas Ypey in 1761, showing the transit of Venus that year.

© United States Naval Observatory

Catching a glimpse of the transit of Venus is rare and has been important throughout the centuries. The next such sightings are predicted for 2117 and 2125. Recording each transit is vital research, which helps scientists to determine the scale of the Solar System.

‘Oh most grateful spectacle, the realisation of so many ardent desires.’

Jeremiah Horrocks, seeing the transit of Venus in 1639

Today Venus has the slowest rotation of any planet in the Solar System, taking 243 Earth days to complete one rotation on its axis. This period is known as the sidereal day, which is different to a solar day – the time it takes for the Sun to return to the same point in the sky. On Earth the sidereal day, at 23 hours, 56 minutes and 4.1 seconds, is very close to the solar day, which lasts pretty much exactly 24 hours. But on Venus the difference between these two periods is much greater. Even though the planet takes 243 days to rotate on its axis when combined with its orbit, a solar day on Venus lasts for 116.75 Earth days. It means every day on Venus lasts almost four months on Earth, and not only that, but Venus also rotates from east to west (one of only two planets to do so, along with Uranus). So across this toxic world a sunrise would last literally for days as it inches across the sky.

This slow progression of the Sun in the Venusian sky, due to the planet’s creeping rotation, has raised many questions about how in the past the planet would have been heated and how the climate would have been affected by such a different rotation compared with the Earth’s. Today the climate of Venus is what is known as isothermal – there is a constant temperature between the day and night sides and between the equator and the poles. This is because the thick atmosphere literally acts like a blanket, dissipating the heat of the Sun so that the only real variation in temperature on the Venusian surface occurs due to differences in altitude. In its past, however, this may have been very different – with a more Earth-like atmosphere, so the Sun would have been beating down on the planet’s surface for days on end.

© NASA / GODDARD SPACE FLIGHT CENTER / SDO / SCIENCE PHOTO LIBRARY

Composite image showing the transit of Venus in June 2012 (in black spots).

© NASA/JPL/USGS

NASA’s Magellan mission in the 1990s sent back images of Venus that enabled scientists to create a more detailed image of the landscapes of this long-lost world. They revealed a terrain of lowlands and highlands, dotted with active volcanos – a far cry from the ancient watery Venus imagined and depicted in some artworks.

© NASA

‘In the GISS model’s simulation, Venus’s slow spin exposes its dayside to the Sun for almost two months at a time. This warms the surface and produces rain that creates a thick layer of clouds, which acts like an umbrella to shield the surface from much of the solar heating. The result is mean climate temperatures that are actually a few degrees cooler than Earth’s today.’

Anthony Del Genio, planetary scientist

To make things even more complex, we know the spin of a planet is intimately linked to its climate and we’ve got strong evidence to suggest that how fast a planet spins is directly related to its chance of habitability. Until very recently it was assumed that the slow rotation of Venus must have been caused by the presence of a thick atmosphere early on in its history that in effect acted as a brake on the planet’s spin. However, recent studies now suggest the planet could have had a thin atmosphere like that of modern Earth and still have ended up with its slow rotation.

Gradually, as we start to build a picture of ancient Venus we begin to see beyond the cloud cover of today through to an ancient planet with an Earth-like atmosphere, and a day lasting over 200 Earth days as the Sun beat down on the ocean-covered surface.

To make sense of the climate of this Earth-like Venus, the team at the Goddard Institute needed to make another tweak (or postulation, to be more precise) to the model. With the Sun hitting the one side of the surface for so much longer than on the Earth, the evaporation rate of the oceans would be far greater and potentially incompatible with the water world we suspect existed, but by simply adjusting the amount of dry land on the surface of Venus, especially in the Tropics, the effect is dramatic. With a higher percentage of land, the models suggest that even the slow rotation would not dry out the planet, and it could have held on to enough water to be ripe for supporting the emergence of life.

By combining all of this data, the GISS team have painted our most up-to-date picture of early Venus, and it’s a beguiling image. Within the infant Solar System, it is a planet the size of Earth with a similar atmosphere to the one we see today. On Venus days lasted for months as the Sun arced slowly across the sky from west to east, rising and setting over a vast, shallow ocean.

Finally, the data from radar measurements taken by NASA’s Magellan mission in the 1990s was used to paint the last brushstrokes of this long-lost world. Filling in the lowlands with water, the topography of this ancient world emerges with the highlands exposed as the Venusian continents. It all points to the possibility that Venus could have been the first habitable world in our Solar System. So what changed? To find out we need to look not just at the planet in isolation but also at the star around which it orbits.

GOODBYE TO LIFE

No planet lives out its life in isolation. Venus, like all the planets is part of a Solar System, a system that is driven more than anything else by the star at its centre. Today the Sun burns bright in our skies, bathing our planet in just enough starlight to keep the oceans from freezing, but not too much to boil them away. Earth lies in the sweet spot we call the Goldilocks zone, but as we have already seen in this chapter, nothing in the Solar System is forever and what we see today is not what we will see tomorrow nor what we would have seen yesterday.

© frans lemmens / Alamy Stock Photo

Capturing the Sun’s warmth is essential for life on a planet, but when too much heat is trapped it can have devastating consequences.

© NASA/JPL-Caltech, Illustrations by Jessie Kawata

David Grinspoon, astrobiologist, on Venusian life:

‘So when we say, as we often do, that Venus is completely uninhabitable, we should put a little asterisk next to that statement, because, we’re talking about the surface environment. But actually if you go up from the surface about 50 kilometres you reach a zone that may be habitable on Venus; in the clouds the pressure and temperature is roughly what it is here on the surface of Earth. There are energy sources in terms of radiation and chemical energy, there are nutrients, there is even liquid water medium – although it’s concentrated sulphuric acid in the clouds – but we now know of organisms on Earth that love concentrated sulphuric acid. So there’s nothing to rule out life in the clouds of Venus and there are even some, I would say circumstantial, facts that suggest the possibility of a biosphere there. I wouldn’t bet on life in the clouds of Venus, but I wouldn’t rule it out until we’ve explored a little more carefully.’

As our Sun gets older, it’s gradually burning hotter and hotter. This is because as it ages the process of nuclear fusion – the fusion of hydrogen into (mainly) helium – gradually leads to an increase in the amount of helium in its core. This rise in helium causes the Sun’s core to contract, which in turn allows the whole star to shrink in on itself, creating an increased pressure that results in a rise in the rate of fusion, and so the energy output of the Sun goes up. If tomorrow the Sun is burning hotter than today, it of course makes sense that in the early days of the Solar System our Sun burned far less brightly. It’s a life cycle that is common to all main-sequence stars, the category of star that includes our Sun, and as the most common type of star in the Universe we have been able to study this life cycle in intimate detail, allowing us to make immensely detailed predictions about the characteristics of our Sun in the past and in the future.

Winding back the clock, the current consensus amongst astronomers is that 4 billion years ago the faint young Sun was at least 30 per cent dimmer than it is today. This cooler Sun would have undoubtedly had a big impact on all of the terrestrial planets. Earth would have been much colder, and as it was receiving far less solar energy it remains something of a mystery as to why our planet wasn’t frozen solid. Instead, at this time on Earth first life was just beginning, in the liquid water that we are pretty certain covered its surface.

At the same time, 3.5 to 4 billion years ago, the young Sun would have bathed Venus in a warmer glow. This ocean world found itself in its very own sweet spot, a world held in a delicate balance. With the Sun weakened and restrained, the Earth-like atmosphere of Venus could act as a gentle blanket, keeping the surface temperate and covered in an abundance of liquid water. But even with this additional solar energy we think Venus would have been cooler than the Earth is today; in fact we believe temperatures at that time would have been like a pleasant spring day here on Earth.

It wasn’t to last. Slowly the young Sun grew brighter, its increased energy output causing temperatures to gradually rise, which in turn began to lift more and more water vapour into the air, thickening the atmosphere and sealing the planet’s fate. Although the oceans of Venus may have persisted for billions of years, as the surface warmed and the atmosphere thickened, the destiny of this planet was already set, driven by an unstoppable process we have recently become very familiar with here on Earth.

The greenhouse effect is a process that has the power both to protect and to destroy a planet, but despite this power it actually boils down to some pretty simple physics. It’s all about how sunlight – solar radiation – interacts with the constituent parts of an atmosphere. In the case of the Earth, as solar radiation hits our atmosphere some of it is reflected straight back out into space, some is absorbed by the atmosphere and clouds, but most of the sunlight (about 48 per cent) passes straight through the atmosphere and is absorbed by the Earth’s surface, where it is heated up. The reason so much solar radiation makes it to the surface is because the gases in our atmosphere, like water vapour and carbon dioxide, are transparent to light in the visible spectrum. When you think about it, that’s pretty obvious because there’s a source of visible light in the sky, the Sun, and we can all see it! But it’s a different story when that sunlight heats the surface of the Earth and re-radiates back out not as visible light but as the longer-wave infrared light – thermal radiation.

© HarperCollins

‘Venus hasn’t stopped heating up, and we believe that as the Sun continues to age, billions of years into the future, it’s going to continue getting hotter. Eventually that means that Earth will go the way of Venus.’

David Grinspoon, astrobiologist

We can’t see this light, but as it radiates back out from the Earth’s surface, carbon dioxide and water vapour absorb the infrared, trapping that energy, and so the planet maintains a higher temperature that is intimately linked to the constituent parts of the atmosphere. The higher its level of gases like water vapour, carbon dioxide, methane and ozone, the greater the greenhouse effect and the bigger the uplift in temperature. Despite the very real threat that this now poses to the future of our planet, the greenhouse effect on its own is not necessarily a bad thing – the Earth would be at an average temperature of around minus 18 degrees Celsius without it – but as we are currently witnessing here on Earth, shift the balance of those gases and things can change very quickly.

At some point in Venus’s past, the levels of water vapour lifted into the atmosphere by the warming sun pushed the greenhouse effect to become more intense. With less and less of the Sun’s energy escaping, the ambient temperatures began to rise exponentially until the day came when the last raindrops fell onto the surface of the planet, the heat evaporating the rains long before they could reach the ground. Venus had reached a tipping point: with the increasing temperatures feeding more and more water vapour into the atmosphere, a runaway greenhouse effect took hold, driving away the oceans. This led to the surface of the planet getting so hot that carbon trapped in rocks was released into the atmosphere, mixing with oxygen to form increasing amounts of another greenhouse gas – carbon dioxide. With no water left on the surface and no other means to remove it, carbon dioxide built up in the atmosphere, setting the planet on a course that would result in the scorched body that we see today.

© NASA

© NASA

In 1977, in the days before computer-generated imaging, NASA commissioned artist Rick Guidice to paint illustrations of the surface of Venus, based on images received from Pioneer probes.

And so Venus’s moment in the Sun came to an end. Earthlings take note: when it comes to the greenhouse effect, there is a precariously thin line between keeping a planet warm and frying it.

THE END OF EARTH?

Of the four rocky worlds, only one has managed to navigate through the instability and constant change of our Solar System over the last 4 billion years and maintain the characteristics needed to support life. Mercury lost its fight early as it was flung inwards towards the Sun, Venus flourished at first, before slowly coming to the boil, and Mars, the runt of the litter, became a frozen wasteland long ago. Only Earth, uniquely amongst the planets, has persisted with an adequate stability over the last 4 billion years to allow liquid water to remain on its surface and an atmosphere just thick enough to keep its climate calm – not too hot and not too cold. Events have rocked us and extremes of temperature have waxed and waned, but never outside of the parameters needed to harbour life. In a chaotic solar system, filled with planetary might-have-beens, Earth is a shining example of stability, and the evidence for this is to be found in every nook and cranny of the planet.

Today Earth is dominated by life; the land and seas are teeming with millions upon millions of species, with thousands of new life forms discovered each year. Somehow, even when disaster threatened, the Earth has remained a living world; while endless species have come and gone, life has always persisted. It’s woven into the fabric of the planet – an integral part of every continent and every ocean. Life plays a crucial role in maintaining the balance of the atmosphere that keeps our planet temperate, but we know for certain it cannot last.

In a chaotic solar system, filled with planetary might-have-beens, Earth is a shining example of stability.

The Kamchatka Peninsula in Eastern Siberia is one of the most inhospitable places on Earth. A volcanic wasteland, peppered with thousands of hot springs, it’s here that we find some of the toughest living things. Extremophiles survive here that are able to withstand temperatures and pH levels higher than any other land-based life forms we have ever discovered. Kamchatka is part of the Pacific ring of fire, and despite its remoteness, biologists have long been enticed here to explore its toxic, bubbling cauldrons for signs of life. Complex life, animals and plants struggle to survive in temperatures above 50 degrees Celsius, so searching for life here is all about searching for single-celled life forms, bacteria and archaea – ancient microorganisms – that are somehow able to endure in this hostile environment. Life forms like Acidilobus aceticus, an archaea that can be found in a hot spring where the water is so acidic it reaches a pH of 2, and where temperatures rise to 92 degrees Celsius. In other parts of the hydrothermal field, bacteria like Desulfurella acetivorans have been discovered, which happily live in pools that are touching 60 degrees Celsius, but it’s these that are the real hotheads. In one of the biggest and hottest pools investigated by scientists, a large number of microbes have been found living in temperatures approaching 97 degrees – making it one of, if not the hottest environment ever studied for signs of life on land.

But to find the greatest hotheads on Planet Earth you need to look not on land but deep beneath the sea. In the furthest depths of the Atlantic, around the black smoker hydrothermal vents blurting out of the ocean floor, we’ve found strains of archaea that can survive temperatures of 122 degrees Celsius, and perhaps even higher.

These rare life forms live at the very edges of biology. Unique adaptations to their cellular chemistry enable the proteins and nucleic acids that create the structure of the microorganism to function, while the membranes that are protecting the cells utilise different fatty acids and lipids to keep the cell stable at the higher temperatures.

© Igor Shpilenok / naturepl.com (http://naturepl.com)